1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for fabricating the device, particularly to a magneto-resistive element disposed in a magneto-resistive random access memory (MRAM) and a peripheral structure.
2. Description of the Related Art
An MRAM is a generic name of a solid-state memory using a magnetization direction of a ferromagnetic body as a recording carrier of information and capable of rewriting, holding, or reading recorded information as occasion demands.
A memory cell of the MRAM has a structure in which a plurality of ferromagnetic films are usually laminated. The information is recorded depending on whether a relative arrangement of magnetization of a plurality of ferromagnetic films forming the memory cell is parallel or anti-parallel in accordance with binary data of “1”, “0”. The magnetization directions of the ferromagnetic films of each memory cell are reversed by a current magnetic field to write the data.
In the MRAM whose practical use has been studied at present, a magnetic tunnel junction (hereinafter abbreviated as MTJ) is used in the memory cell. This structure has been described, for example, in “IEEE International Solid-State Circuits Conference 2000 Digest Paper”, TA7.2. The MTJ comprises a three layer film mainly formed of a ferromagnetic layer/insulating layer/ferromagnetic layer, and a tunnel current flows through the insulating layer. A resistance value of the junction changes in proportion to a cosine of a relative angle of magnetization of both ferromagnetic layers. Moreover, the resistance value of the junction takes a maximum value in a case where the directions of magnetization of both the ferromagnetic layers are anti-parallel. This is a tunnel magneto-resistive effect. One type of the MTJ has a structure that retains data utilizing a difference in magnetic coercive force between both ferromagnetic films. Another type of the MTJ has a so-called spin valve structure in which an antiferromagnetic layer is disposed adjacent to one of the ferromagnetic films to fix the direction of the magnetization for a purpose of improvement of a magnetic sensitivity or reduction of a write current. This structure is described, for example, in “Japanese Journal of Applied Physics”, 1997, No. 36, p. 200.
However, in general, since a material for use in forming an MTJ element has a low vapor pressure, RIE having a high power is used in manufacturing process. Therefore, a charge damage with respect to the MTJ element raises a serious problem.